1. Field of the Invention
The present invention refers to a measuring head for use in radiant energy flash measuring of the thermal diffusivity of heterogeneous samples, in particular of heterogeneous highly radioactive samples or other samples for which remote manipulation is required.
2. Description of the Related Art
From W. J. Parker, R. J. Jenkins, C. P. Butler and G. L. Abbot, J. Appl. Phys. 32(9), 1679-1684 (1961) the so-called "flash method" for measuring the thermal diffusivity "a" is known. In this method the front face of a small disc-shaped specimen or sample is subjected to a very short burst of radiant energy coming from a laser or a flash lamp. The method employs irradiation times in the order of one millisecond. The resulting temperature rise of the rear surface of the sample is measured and recorded, and then thermal diffusivity values are computed from temperature rise versus time.
The simplest and most frequently used way to calculate thermal diffusivity is to use t.sub.1/2 as a characteristic time, i.e. the time needed for the rear side temperature to reach 50% of its maximum value: EQU a=0.1388L.sup.2 /t.sub.1/2 (m.sup.2 /s)
From R. E. Taylor and K. D. Maglic, in: Compendium of Thermophysical Property Measurement Methods 2 (K. D. Maglic, A. Cezairliyan and V. E. Peletsky, eds.) 281-314, Plenum Press, New York (1991) a method for carrying out a transient temperatur measurement is known. The detector for measuring the transient temperature may be a thermocouple, a infrared detector or an optical pyrometer. The detector must be capable to record 0.1 degree change above the ambient temperature. The response time of the detector/amplifier combination must be less than 10% of t.sub.1/2.
FIG. 4 illustrates schematically the known method for measuring the thermal diffusivity.
The prior art measuring heads for carrying out the above described method for measuring the thermal diffusivity are used in connection with lasers as a radiant energy generating means and a detector means for detecting a temperature rise at the rear side of the sample. They comprise a sample holder means for receiving the sample to be measured. However, they are all constructed in view of producing and analysing a pre-fixed type of temperature spike. They in particular do not offer the opportunity to align the sample with the probe beam generated by the laser. Thus the energy shot cannot be deposited on the most suitable area, dependently on the given sample features, and a local axial diffusion measurement cannot be performed.